1. Field of the Invention
This invention relates to a filter adjustment apparatus and method and more particularly to a device and method for adjusting a filter connected in series with a signal processing circuit or a filter connected in series with at least one other filter.
2. Description of the Prior Art
In a checking process for electronic circuits in general, it may become necessary to adjust the peak frequency, dip frequency or the cut-off frequency of the filter circuit to a prescribed target value. Above all, in a circuit formed in an analog integrated circuit or analog IC, while the circuit elements such as transistors, resistors or capacitors can be fabricated with a highly accurate relative ratio of the rated values thereof, the absolute values or magnitudes of these rated values vary from circuit to circuit. Hence, the aforementioned adjustment is thought to be indispensable in a filter circuit wherein a demand is raised for high accuracy.
In general, when carrying out the filter adjustment, the filter output is detected while the frequency of the input signals to the filter is continuously changed, by so-called sweeping, for finding the portions proper to the filter characteristics, such as the peak or dip on the frequency characteristic curve, and the frequency characteristics are changed until the frequency thereof coincides with the prescribed target values.
It is noted that, when adjusting the cut-off, peak or dip frequencies of a filter connected in series with one or more other filters or a filter connected in series with a signal processing circuit, the frequency characteristics of the series circuit including the filter will appear as the combined characteristics of the respective filters or as the combined characteristics of the filter and the signal processing circuit, so that it becomes difficult to check for portions proper to the filter characteristics, such as the aforementioned peak, dip or cut-off points. Above all, when the series filter circuit is provided in an integrated circuit, it is almost impossible to input or output signals into or from the respective filters or the signal processing circuit. In addition, since the adjustment of the respective filters is performed simultaneously by the common adjustment control signal, such that the frequency characteristics of the respective filters are changed simultaneously, the combined frequency characteristics are changed in a complicated way, so that it becomes more difficult to locate the portions proper to the filter characteristics, such as the aforementioned peak points. With increased difficulties in locating the characteristic portions, filter adjustment accuracy is undesirably lowered, while the labor and the time involved in the adjustment are increased.